Conventional pH electrodes usually contain a solution of potassium chloride and a pH buffer, to maintain the pH value, in a glass sheath. This fill solution, together with the pH sensing glass membrane formed in the sheath, defines the potential that is measured to determine the pH of the sample solution in which the electrode is immersed. The potential produced is the sum of the potential developed across the glass membrane and the potential of the internal reference electrode, usually a coated silver wire. The membrane potential is given by: ##EQU1## where pH.sub.inner and pH.sub.outer are the pH values of the solutions inside and outside the membrane respectively, R is the Universal Gas Constant,
T is the temperature of the electrode system in .degree. K, PA1 n is the charge on the ion, i.e. the number of electrons transferred in the reaction, and PA1 F is the Faraday Constant; and the reference electrode potential is given by: ##EQU2##
In most conventional electrode systems, the fill solution has an excess of chloride ions, and the silver ions present are derived from solid silver chloride that is attached to the electrode and/or is in solution. Thus, the activity of the silver is in equilibrium with the chloride activity via the solubility constant of silver chloride.